Posted by: recordingsofnature | September 13, 2011

Experiments with new horn shapes

This is a little report on the results from experimenting with new pressure equalizers made of FIMO polymer clay.

FIMO clay form 2 pressure equalizer

FIMO clay form 2 pressure equalizer

Instead of a bundle of toothpicks or matches, these horns are now made of solid hobby FIMO clay, however I have tried to keep the geometries similar to those giving good results with the sticks bundle. I was curious to find out about the effect of changing the material from wood sticks to hobby clay.

It was quite challenging to shape the hobby clay into a precise form, and ensure that the two horns were the same for a stereo pair. The clay feels a little soft and flexible to form but luckily it kept the shape well once formed and afterwards in the oven.

During the fabrication process:

FIMO clay horns being formed

FIMO clay horns during forming process before hardening in the oven at 100C in 45 min.

So here are the results:

Form 1

The ‘form 1’  geometry is very similar to the one described here. To check the frequency response, the horns were characterized with a simple on-axis test (see last section). The results are shown on the chart below. It was again seen, that fine adjustment of the microphone tip (dD) of just 0.5 mm gives a clear HF rise. It is a very reproducible effect seen in all the following measurements.

form 1 fimo pressure equalizer

form 1 fimo pressure equalizer

Form 2

The ‘form 2’ is a ‘milder’ version with less effect:

form 2 fimo clay pressure equalizer

form 2 fimo clay pressure equalizer

Optimal toothpick form

For reference, here is a new measurement of the old “optimized” shape described in the previous post. The gain curve appears more bumpy than the two previous ones.

sticks bundle pressure equaliser

sticks bundle pressure equaliser

Comparing all the three horns gives some overview and selection guide. It is mainly the HF rise in the 4-10kHz region that differs amongst the geometries. There are also some clear differences from 25kHz and up. It seems the solid FIMO clay material has a stronger effect than the sticks bundle with the same geometry. With the horns and fine adjustment of the microphone tip I think there are possibilities for various HF rises for semi/far distance recordings.

Comparison of candidate semi far distance pressure equaliers

Comparison of candidate semi far distance pressure equalizers

Measurement method

I did not make the full directional measurement at this time, -as it requires the whole living room to be covered with sound absorbing material as well as well as timely elimination of a long list of noise sources in the apartment…

So instead, this is a quite fast and useful method to determine theon-axis frequency effect of a horn modified microphone. Two microphones are placed side by side (15 cm apart) recording a noise source placed 1 meter right in front. A crisp and thin plastic bag is a useful noise source. Initially, both microphones should be unmodified, to obtain the calibration frequency curve. Then only one of the microphones is modified with a horn and the recording is repeated. Now by subtracting recorded spectral curves from each microphone, and taking into account the calibration curve, the gain of the horn will show up. I am using a old version of Cool Edit pro  for the spectral analysis.

Calibration curve:

Calibration curve, no horns

Calibration curve, no horns

Example of curve with one horn fitted:

Frequency curve with Form 1 fitted on right

Frequency curve with Form 1 fitted on right



  1. Thanks for sharing your latest results – have just downloaded your previous pdf files. Will get back to you sometime later, as I want to run some calculations based on the geometry of the horns and the various frequency response results.


  2. At last had chance to look over the data – here are some of my findings – hope they make sense!


  3. […] For this recording, the distance was about 20-25 cm. As usual, the black pressure equalisers (fimo form 1, 1mm) serves to enhance pickup in the 5-9 khz band and directionality of the omni microphones (like the […]


  4. […] the forward directionality and adds an about 4dB presence gain in the 5-8kHz range (also described here). This is also seen from the measurement of the frequency response of the setup, as seen […]


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